The present invention relates to a method of interconnecting conductors, and more specifically, to a method utilized for electrically interconnecting an electrical component, such as an IC package, having a plurality of conducting terminals to a printed circuit board (PCB).
A conventional method for interconnecting an IC package to a PCB is to use soldering techniques. Since the pins of the IC package are arranged with very close spacings, 30 mil in general, precise dot deposition is required to place solder only upon the pins and not therebetween. Furthermore, soldering techniques require temperatures high enough to melt the solder so that both the IC package and the printed circuit board must be able to withstand such high temperatures.
A number of methods therefore have been proposed to eliminate the use of solders in electrically interconnecting IC packages to PCBs. A U.S. Pat. No. 4,820,376 issued to Lambert et al, discloses the use of a conductive polymer interconnect material (CPI) having a plurality of chains of electrically conducting particles. A sheet made of the CPI material can be disposed onto the surface of a PCB. When the disposed sheet is pressed by the pins of an IC package, the conducting particle chains therewithin electrically connect the pins to the PCB. Since the conducting particles chains are randomly distributed and are also fixed within the CPI material, they can not be moved so as to be gathered at specific positions.
In U.S. Pat. Nos. 4,667,401, 4,720,740, and 4,868,637 issued to Clements et al; there are disclosed the use of an adhesive of dielectric polymeric resin having a viscosity in the range of 10,000 cps to 40,000 cps and 20 wt % to 25 wt % conductive metal particles. The adhesive is coated uniformly onto the surface of a PCB and afterwards an IC package is pressed onto the coated layer so that portions of the adhesives that are distributed on the conductive strips of the PCB are pressed by the pins of the IC package. The pressed portions are thereby reduced in volume such that the concentrations of conductive metal particles at these points are increased to 45-50 percent by weight. In accordance with a TABLE 1 given in the specification of U.S. Pat. No. 4,667,401, the resistance value of an adhesive having 20 wt % to 25 wt % conductive metal particles is greater than 10.sup.14 .OMEGA. and that of adhesive having 45 wt % to 50 wt % conductive metal particles is less than 1.OMEGA.. As a result, although the pins of the IC package are submerged into a same layer of adhesive, electrical connections are established between the pins of the IC package and the conducting strips of the PCB and while there are still excellent insulations among the pins of the IC package.
The method according to the patents of Clements et al; however, suffers from high failure rate of electrical connections since the establishment of an electrical connection requires that the concentration of conductive metal particles exceed a threshold value (somewhere between 40 wt % and 45 wt %), the denser the better. To concentrate the conductive metal particles merely by pressing the adhesive is somewhat a rough method that can not guarantee high successful rates of electrical connections.
Jin et al, in their U.S. Pat. No. 4,737,112 disclose an electrical interconnection which is established by means of an anisotropically conductive, composite layer medium comprising electrically conductive particles in a nonconductive matrix material. Magnetic field is applied to align the conductive particles to form electrically conducting chains.
Hutcheson in his U.S. Pat. No. 4,170,677 dislcoses a method of making electrical connections between large-scale integrated circuit boards or the like using a plastic adhesive charged with conductive particles. The method includes the steps of: coating the board with a conductive-whisker charged plastic adhesive, placing another board on the adhesive coating; applying an aligning field to make the conductive whisker align perpendicular to the boards, and hardening the adhesive while the field is applied. However, as can be seen from FIG. 2 of Hutcheson's patent, applying magnetic field can only make the conductive whiskers align perpendicular to the boards, but not gather. Therefore, the method of Hutcheson is the same as that of Jin et al; and can not guarantee high successful rates of electrical connections.